Vibration Monitoring
1.Examples vibration monitoring & maintenance of machinery.
2.How to use vibration analysis for predictive maintenance.
3.Practical aspects & specific techniques. 4.Focus on rotating
machinery. Unbalance Misalignment Gear tooth defects Bearing
defects 5.Related techniques.
1. Examples
Definition of vibration monitoring:
Regular monitoring of machinery vibrations undertaken as part of
a Predictive Maintenance Program. Readings are compared with past
levels, with significant change as an indicator of developing
machinery faults.The objective is to provide valuable lead-time for
maintenance planning. A comprehensive monitoring program usually
includes vibration analysis.
Condition Monitoring Techniques:
Vibration analysis Oil analysis Wear particle analysis
Temperature monitoring. Ultrasonics Infrared thermography
Performance evaluation
Examples:
Car suspension Crash Tests Aircraft wing tests Bridge vibration
Machinery
2.How to use vibration analysis
Maintenance philosophies:
The main value of Predicted Maintenance is to allow convenient
scheduling of corrective maintenance, and to prevent unexpected
equipment failures. The key is "the right information in the right
time". By knowing which equipment needs maintenance, maintenance
work can be better planned (spare parts, people, etc.) and what
would have been "unplanned stops" are transformed to shorter and
fewer "planned stops", thus increasing plant availability. Other
advantages include increased equipment lifetime, increased plant
safety, fewer accidents with negative impact on environment, and
optimized spare parts handling.Advantages of Predictive
Maintenance:
1.Eliminate unnecessary disassembly. New bearings sometimes fail
early. Poor assembly. 2.Reduce unscheduled downtime. 3.Avoid
wrecks. 4.Reduce insurance costs.
Types of Vibration Monitoring:
1.Continuous monitoring Permanently installed transducers.
Costly. 2.Periodic monitoring More cost-effective. 3.Trending
Compare data with a baseline. Long-term trends for each machine
more practical.
3.Practical aspects
Vibration monitoring entails the regular monitoring of the
vibrations of machinery as part of a predictive maintenance
program. Vibration readings are compared with past levels, with
significant change as an indicator of developing machinery faults.
The objective is to provide valuable lead-time for maintenance
planning. A comprehensive monitoring program usually includes
vibration analysis.
Classifications of Machines:
1.Critical: High safety, operational consequences. High cost to
repair. Optimised operation save energy. 2.Essential: Moderate
operational consequences and repair cost. Medium horsepower.
3.General: No criticality. Low repair cost, secondary damage
minimal
Assigning Maintenance Strategies:
1. Critical Machinery: Proactive, Predictive
2. Essential Machinery: Preventive, Predictive (lesser
sophistication)
3. General Purpose Machinery: Breakdown, Predictive (portable
equipment)
How often?
Machines with a known history of problems should be monitored
daily. Most machines monthly. Machines with a proven track record
only quarterly. When problems arise, monitor more frequently.
Under what conditions?
1. Always the same 2. Try to monitor at full load. 3. The same
speed, or implement advanced techniques to compensate for speed. 4.
A 10% speed difference is acceptable in some cases such as fans and
pumps.
Where to take readings?
Depends on failures that are monitored. Most often axial and
radial readings on each bearing housing. Mark positions (washer for
magnet, tape, pen). Transducer must be perpendicular to surface.
Can use a tri-axial sensor for difficult to reach areas.
Accelerometer Mounting:
1.Order of preference: Stud mounting Adhesive Magnetic base Hand
held probes or stingers 2.Stud mounted pickups provide maximum
frequency response. 3.Sensor resonance is influenced by mounting
method.
Fourier Transform:
An FFT consists of: 1. Amplitude 2. Phase
Parameters to choose when measuring:
1.Displacement: 1.Frequency range is below 10 Hz 2.Velocity:
1.Frequency range from 10 Hz to 1 kHz 3.Acceleration: 1.Above 1
kHz
4. Rotating Machinery
Common Machinery Faults:
Unbalance: Static
Amplitude due to unbalance will vary with the square of
speed.
The FFT will show 1x rpm frequency of vibration. It will be
predominant. Phase difference is as shown.
Bent Shaft:
> For a shaft bend near the centre: A frequency of 1xrpm is
predominant.> Bend at ends: 2xrpm is predominant> No phase
difference in radial direction at one location.> 180 phase
difference in axial plane
Misalignment:
After unbalance, misalignment is the major cause for high
vibrations.
Two kinds of misalignment: Angular - shaft ends meet an angle.
Parallel - shaft ends are parallel but have an offset.
1. Angular Misalignment
1. Predominant peak in the frequency domain is at 1 rpm. 2. 1 x,
2 x, 3 x may be present. 3. High axial vibration with 1 and 2 . 4.
Axial phase difference across the coupling is 180.
2. Parallel Misalignment
1. The parallel misalignment can be into horizontal and vertical
misalignment. Horizontal misalignment is misalignment of the shafts
in the horizontal plane: i.e. a motor shaft is moved horizontally
away from the pump shaft, but both shafts are still in the same
horizontal plane and parallel. Vertical misalignment is
misalignment of the shafts in the vertical plane: i.e a motor shaft
is moved vertically away from the pump shaft, but both shafts are
still in the same vertical plane and parallel.
2. The phase difference in radial direction across the coupling
is 180. 3. The predominant peak in the frequency domain is at 2 x
rpm. 4. Vibrations in radial direction are higher than in the axial
direction.
Gear tooth defects:
Gear tooth wear:1. Excitation of gear natural frequencies. 2.
Sidebands are spaced at the running speed of the bad gear. 3.
Sideband amplitudes rise with wear.
Bearing defects:
Cocked Bearing Symptoms: Vibration symptoms very similar to
direct drive angular misalignment. High axial vibration @ 1x rpm,
harmonics at 2x & 3x. 2x rpm radial component often as high or
higher than 1x component. Axial phase shift around the face of the
bearing equal to change in transducer location
Balancing:
1.Can be done with a vibration analyzer or a balancing machine.
2.Performed in a vacuum chamber. 3.Typically, rotors up to 8 tons,
up 1.7 meters in diameter, at speeds of up to 60 000 rpm.
Alignment method: Laser
1.The laser alignment system comprises an analyser and two laser
heads. 2.The fastened laser heads face each other. Each head has an
emitter and a receiver for lasers. 3.As the shafts are turned, the
receivers trace the shifts in laser beams. 4.These are communicated
to the analyser and computations are done to correct the
situation.
5. Related techniques
1.Vibration Analysis and Oil / Particle Analysis are the main
PdM techniques. 2.There are also other standard techniques like:
1.Ultrasound 2.Thermography 3.These are useful tools in certain
applications. 4.Complement the main techniques.
Thermography:
The use of thermograms to study heat distribution in structures
or regions, for example in detecting tumours.
Thermography can be used to detect;
1.Misalignment. 2.Defective compressor valves. 3.Insufficient
lubrication. 4.Bad bearings, gears, belts, belt slippage, clutches,
chains. 5.Leaking valves, blocked pipes. 6.Tank levels.